msm: vidc: optimize buffer tag allocation

- Possiblilty of performance hit due to memory
  alloc/free at frame level.
- Reuse same buffer_tag instead of reallocating.

Change-Id: Ifb8efc43907feea4d6da5e8a0bf0aeaa47c8e1d7
Signed-off-by: Govindaraj Rajagopal <grajagop@codeaurora.org>